1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic apparatus using a programmable patterning means, several additional calibrations, as compared to a machine using a conventional mask, must be carried out. For example, it is necessary to measure the intensity of the radiation delivered to the substrate for each element (e.g., pixel) of the programmable patterning means. The size and position of each spot projected onto the substrate must also measured. Given the very large number of elements in a programmable patterning means, potentially tens of millions, such pixel-by-pixel calibrations take an excessive length of time and represent a substantial reduction in throughput if it is necessary to repeat them periodically.
In a lithographic apparatus for forming very fine patterns, particularly one employing a reducing projection system, the calibration is also very difficult as the spot projected onto the substrate for each pixel is extremely small, much smaller than any detector, even the individual elements of a charge coupled device (CCD). It should be noted that the spot size on the substrate is determined by the projection optics and may be larger than the geometric image of a single pixel of the programmable patterning means, depending on the value of a k1 factor. K1 is a process factor depending on the coherence of a light source. For example, K1 is 0.25 and 0.5 for incoherent and coherent illumination, respectively.
Carrying out a series of test exposures and measuring the results can be done, but is very time consuming, and hence impracticable for periodic recalibration. It has therefore been proposed to carry out calibration measurements at an intermediate image plane of the projection lens, e.g., by providing a semi-transparent mirror to direct a portion of the beam to a detector or by providing a detector that can be moved into and out of the beam. For examples of this former method see U.S. Published Patent Application 2003/0081303 A1 and WO 03/046665, which are both incorporated by reference herein in their entireties. An intermediate image plane where the image is much larger than that projected onto the substrate can be chosen. However, a semi-transparent mirror will inevitably degrade the projected image to some extent and providing the space and mechanism for a moveable detector in the projection lens may not be convenient. Also, measurements at an intermediate image plane cannot take account of the effects of subsequent elements of the projection lens.
U.S. Pat. No. 6,121,626 and US 2001/0033996A disclose lithographic apparatus using a transmissive dynamic mask and a CCD mounted on the wafer stage, which are both incorporated by reference herein in their entireties. The CCD image is compared to the desired image to optimize the pattern on the dynamic mask as well as focus, dose, numerical aperture (NA), and σ settings.
Therefore, what is needed is an apparatus and method that allow for more rapid and reliable calibration of a programmable patterning device, for example using measurements made at substrate level.